Expandable liner hangers are generally used to secure a liner within a previously set casing or liner string. These types of liner hangers are typically set by expanding the liner hangers radially outward into gripping and sealing contact with the previous casing or liner string. Many such liner hangers are expanded by use of hydraulic pressure to drive an expanding cone or wedge through the liner hanger.
The expansion process is typically performed by means of a running tool or setting tool used to convey the liner hanger and attached liner into a wellbore. The running tool or setting tool may be interconnected between a work string (e.g., a tubular string made up of drill pipe or other segmented or continuous tubular elements) and the liner hanger.
If the liner hanger is expanded using hydraulic pressure, then the running tool or setting tool is generally used to control the communication of fluid pressure, and flow to and from various portions of the liner hanger expansion mechanism, and between the work string and the liner. The running tool or setting tool may also be used to control when and how the work string is released from the liner hanger, for example, after expansion of the liner hanger, in emergency situations, or after an unsuccessful setting of the liner hanger.
The running tool or setting tool is also usually expected to provide for cementing therethrough, in those cases in which the liner is to be cemented in the wellbore. Some designs of the running or setting tool require a ball or cementing plug to be dropped through the work string at the completion of the cementing operation and prior to expanding the liner hanger.
In running tools or setting tools that expand a liner hanger using hydraulic pressure, multiple stacked pistons may be employed to apply force to an expanding cone or wedge to drive it through the liner hanger. The force required to expand the liner hanger may vary widely due to factors such as friction, casing tolerance and piston sizing. In addition, the pistons may be exposed to internal pressure in the tool during cementing of the liner and/or release of a cementing plug and/or circulation of drilling fluids through the liner and the wellbore, thereby risking premature expansion of the liner hanger. Accordingly, hydraulic pressures in the tool must be carefully monitored during activities undertaken prior to expanding the liner hanger.